+1 from me as I too have been bitten by this.
On Wed, Apr 10, 2013 at 2:24 PM, Levent Erkok <erkokl at gmail.com> wrote:
> The current definition of "abs" and "signum" for the types Float/Double is
> not IEEE-754 compliant. To wit:
>> Prelude> abs (-0.0::Float)
> -0.0
> Prelude> signum (-0.0::Float)
> 0.0
>> The correct result should be the other way around; abs returning 0.0 and
> signum returning -0.0 when they receive a negative-zero. The same also
> holds for the type Double.
>> The issue came up several times in several forums, with general consensus
> that the behavior should match the IEEE-754 specs. Here're three different
> discussions on this matter:
>>>http://www.haskell.org/pipermail/libraries/2011-January/015761.html>>http://stackoverflow.com/questions/10395761/absolute-value-of-negative-zero-bug-or-a-part-of-the-floating-point-standard>>http://www.haskell.org/pipermail/haskell-cafe/2013-April/107471.html>> Proposed fix: Section 6.4.4 of the report
>http://www.haskell.org/onlinereport/basic.html#sect6.4 gives "default"
> definitions for abs/signum; which fails to take into account of
> negative-zero values for floating-point types. An easy fix would be to add
> to the report a note on the status of negative-zero for Real/Float
> instances; with individual implementations explicitly checking for
> negative-0 first. For instance GHC's implementation can be changed as
> follows:
>> instance Num Float where
> signum x | isNegativeZero x = x
> | x == 0 = 0
> | x > 0.0 = 1
> | otherwise = negate 1
> abs x | isNegativeZero x = 0
> | x >= 0 = x
> | otherwise = negate x
>> A similar change would need to be done for the "Num Double" instance as
> well. Of course, implementations can take advantage of the underlying CPU's
> native floating-point abs/sign functions if available as well, avoiding
> explicit tests at the Haskell code; based on the underlying platform.
>> Library guidelines suggest a discussion period of 2 weeks on this issue.
> I'm hoping that we can resolve the issue in a timely manner, and at least
> GHC's implementation can match the desired semantics in the next release.
> If there's consensus at the end of 2-weeks; I'll go ahead and create a
> corresponding ticket for GHC.
>> Thanks,
>> -Levent.
>>> _______________________________________________
> Libraries mailing list
>Libraries at haskell.org>http://www.haskell.org/mailman/listinfo/libraries>>-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/libraries/attachments/20130410/70d62df0/attachment.htm>